Approved Documents set out what may be accepted as reasonable compliance with the relevant Building Regulations in England to which they refer.  Following the guidance in an Approved Document establishes a presumption of compliance with the requirements covered by the guidance. However, compliance is not guaranteed.  The guidance may not apply if the particular case is unusual in some way.

Approved Document L1B covers the energy performance requirements for existing dwellings.  Further information and free pdf versions of the Approved Documents can be found on the Gov.UK website.